The type of fuel cell that the present invention concerns is disclosed in International patent applications PCT SE2007/050222 and in PCT SE2005/001514.
Fuel cells of this type typically consist of the following design features/functionalities:
1) A sealing functionality creating the anode gas chamber. This is accomplished by using an adhesive which thereby seals the Membrane Electrode Assembly (MEA) to an anode current collector foil.
2) A gas distribution functionality to distribute the hydrogen gas to different cells in a fuel cell device. This is accomplished by forming a support plate with gas channels for the hydrogen gas. The fuel cells are attached to the support plate by adhesive and/or clamping means. From the support plate there are holes leading to the anode gas chamber of each cell.
3) An electrical interconnect functionality which collects the current from one cell and distributes it to the adjacent cell, preferably with minimal resistance and in such a manner that uniform current density is obtained over the active area of the cells.
4) A clamping feature. By subjecting the fuel cell to a clamping force the internal resistance within the cell is decreased, i.e. contact resistances between different materials and specific resistances inside materials (e.g. by compressing the Gas Diffusion Layer (GDL) its fiber-fiber connections improves). Analogous to the electrical contact also the heat conductivity is improved by the clamping and thereby more heat can be dissipated from the reaction layers (i.e. the electrodes). The clamping feature is closely linked to the electrical interconnect functionality.
All these design features/functionalities applied together form a fuel cell device.
A general problem with fuel cell assemblies (fuel cell devices) is that fuel feed is not always optimal (i.e. constant and corresponding to the power demand of a device being powered by the fuel cell power source) and therefore one has to control either the fuel feed (hydrogen gas flow) to or the power draw from the fuel cell device.
Often times, fuel cell stack performance is monitored by detecting the voltage of individual cells or groups of cells in the stack. A typical stack generally comprises 30 to 200 individual cells. Voltage detection of individual cells or groups of cells is expensive and requires a complex data acquisition system and control algorithm to detect and identify a voltage condition outside a preset voltage range and to take corrective action or shut down the stack until normal operating conditions (i.e. conditions within a desired or preferable range) can be restored. A typical approach to monitoring fuel cell performance using voltage detection is described in U.S. Pat. No. 5,170,124. This patent describes an apparatus and method for measuring and comparing the voltages of groups of cells in a fuel cell stack to a reference voltage.
If the measured and reference voltages differ by more than a predetermined amount, an alarm signal or process control procedures can be initiated to implement a shut-down sequence or commence remedial action. While this voltage detection approach identifies the existence of an out-of bounds condition, the approach is imprecise as to the source and/or nature of the problem which triggered the out-of-bounds condition.
In WO 00/02282 (Ballard Power Systems) there is disclosed an electrochemical fuel cell stack includes a plurality of fuel cells. At least one of the fuel cells is a sensor cell. The sensor cell has at least one structural dissimilarity with respect to the remaining fuel cells of the plurality. The structural dissimilarity may include, for example, a reduced sensor cell electrochemically active area, reduced electrocatalyst loading, modified anode or cathode flow field, different electrocatalyst composition, or a modified coolant flow field configuration. The sensor cell operates under substantially the same conditions as the remaining cells in the stack. However, in response to a change in a particular stack operating condition, an electrical or thermal response, preferably a voltage change, is induced in the sensor cell which is not simultaneously induced in the remaining fuel cells. Thus, the sensor cell can detect undesirable conditions and its response can be used to initiate corrective action. More than one sensor cell, specific to different types of conditions, may be employed in the stack. In the absence of undesirable conditions, the sensor cell can function as a power-producing fuel cell.
According to '282 sensor cells incorporated in a stack can also serve as useful power-producing cells. Thus, during operation of the stack to produce electrical power the sensor cell(s) and the remaining cells are connected to provide electrical power. A variable electrical load may be applied across the fuel cell stack comprising the sensor cell(s). Sensor cells according to '282 are connected in series in the stack.